As used in the art, the term "resole" refers to phenolic resins that contain useful reactivity, as opposed to cured resins. At this stage, the product is fully soluble in one or more common solvents, such as alcohols and ketones, and is fusible at less than 150.degree. C. Phenol-aldehyde resole resins are generally prepared by reacting a phenol with an excess molar proportion of an aldehyde in the presence of a basic catalyst, such as an alkaline catalyst or an amine catalyst.
Phenolic resole resins are typically made by condensation polymerization of phenol and formaldehyde in the presence of a catalyst at temperatures between 40.degree. C. and 100.degree. C. Due to the low yield of the phenol and formaldehyde condensation under the reaction conditions that are normally used, a typical resole resin contains a high percentage of free monomers, i.e., phenol and formaldehyde. These free monomers are volatile and highly toxic. Reducing the level of free monomers in such resins, thus reducing their emissions into the environment during application processes, has been one of the most heavily researched areas by both phenolic resin producers and resin users for many years. The catalysts commonly used in phenolic resin production are sodium hydroxide and triethylamine ("TEA"). TEA is very volatile and toxic. Its emission into the atmosphere is regulated by government agencies.
Phenolic resins are widely used as binders in the fiberglass industry. Most resins for the fiberglass industry are catalyzed with inorganic catalysts because of their low cost and non-volatility. When an inorganic base-catalyzed phenolic resin is mixed with urea solution (known in the art as "premix" or "pre-react"), certain components of the phenolic resin, known as "tetradimers," crystallize out, causing the blockage of lines, interrupting normal operations, and reducing resin use efficiency. The crystallized material is difficult to dissolve and hinders uniform application of the resin to the glass fiber. Due to the very poor tetradimer stability of the premix solutions of inorganic base-catalyzed resins and urea, vigorous precautions must be taken with the inorganic base-catalyzed resins, to avoid tetradimer crystal growth, for example, by regular cleaning of the storage tanks and lines, and by shortening the time between the preparation and use of the premix solution.
Phenolic resins catalyzed with an organic catalyst such as TEA are especially useful for applications where high moisture resistance and higher physical strength are required. When a phenolic resin such as phenol-formaldehyde resin ("PF") catalyzed with an organic catalyst is mixed with an amino resin such as urea-formaldehyde resin ("UF"), the resultant PF/UF or PF/U is expected to be much more storage stable and to have much less tetradimer precipitation or crystallization since the organic catalyst, unlike an inorganic base, will increase the solubility of the phenolic resin in the PF/UF solution. PF/UF mixture or premix is often used as a binder in the fiberglass industry. However, the benefit of a storage-stable, highly moisture-resistant and high quality premix system with a typical organic catalyst such as TEA cannot be realized due to its high volatility and toxicity.
A typical phenolic resin to be used as a binder for fiberglass is made at a formaldehyde/phenol mole ratio as high as six to virtually eliminate free phenol in the resin. The high formaldehyde/phenol ratio required to achieve the very low free phenol concentration results in free formaldehyde concentrations as high as 20%. The high percentage of free formaldehyde in the resin must be scavenged by the addition of a large amount of urea or any other formaldehyde scavengers.
In the fiberglass industry, a phenolic resin is normally produced by a resin manufacturer, and then is sold to a fiberglass producer. The addition of urea to the phenolic resin to form UF resin is done in the fiberglass plant. When the urea is added in a fiberglass plant, the level of free formaldehyde is reduced to about 0.5-1.5% after the PF/U premix is allowed to react at room temperature for a few hours. Often urea cannot be added to the phenolic resin in the resin manufacturer's site because the mixture of phenolic resin and urea (the "premix") is not stable enough to permit it to be stored for two to three weeks without tetradimer precipitation. Consequently, most phenolic fiberglass resins are sold without any added urea.
If it were possible to produce a phenolic resin which is highly storage stable without any precipitation for two to three weeks, without the use of a volatile and toxic catalyst, it would be possible to add the formaldehyde scavenger in the resin producer's site. Such a PF/UF premix system would be attractive to fiberglass manufacturers because it would reduce storage facility requirements, as well as the manpower needed for handling, maintaining and purchasing the materials used in the premix. Perhaps most importantly, such a premix system would eliminate the need to handle materials with free formaldehyde concentrations as high as 20%, and formaldehyde emissions would also be reduced.
If the resin manufacturer adds all the urea to the phenolic resin in a premix system, it is normally many days before the resin is used in the fiberglass plant. During this time, virtually all the free formaldehyde in the resin reacts with the added urea. By the time the premix is used in the fiberglass plant, the free formaldehyde content in the premix can be as low as 0.1%. Thus, the use of such a ready-for-sale premix system reduces the emissions of free monomers.
A ready-for-sale premix system can also reduce the free monomer concentrations in the resin and attendant emissions by permitting further advancement of the resin polymerization. Since the PF/U premix of a typical resin presently available in the art has poor storage stability, the phenolic resin is often slightly under-cooked to maintain low molecular weight and acceptable storage stability. A resin catalyzed with an organic catalyst may still have excellent storage stability at a slightly higher advancement or higher molecular weights. Therefore, it will be possible to advance the resin to lower the contents of free monomers in the resin, which in turn reduces monomer emissions.
In summary, tetradimer precipitation and emissions of phenol, formaldehyde and amine remain the most important issues in the phenolic fiberglass industry. Although most widely used, inorganic base-catalyzed resins have poor tetradimer stability, low moisture resistance, low physical strength, and are not suitable for ready-for-sale premix production. Resins catalyzed with an organic catalyst should have excellent storage stability when mixed with a high level of urea, high moisture resistance, high physical strength, and permit ready-for-sale premix production by the resin manufacturer, but a typical organic catalyst such as TEA being highly volatile and toxic may not be suitable for a wide range of applications.